Activation of thrombin and factor Xa (fXa) occurs on vascular and biomaterial surfaces and initiates coagulation, platelet, signaling and cell proliferation reactions, which may then promote pathological thrombosis, occlusion and restenosis in the native circulatory system and on surfaces of implanted vascular grafts, stents, hemodialysis access grafts and ventricular assist devices. Early intervention via the neutralization of thrombin/fXa molecules as they are generated at blood-surface interfaces would be a widely applicable strategy for reducing thrombin/fXa mediated pathologies. Implementation of this approach will require blood-borne inhibitors to target vascular and biomaterial surfaces and to remain stable under focal and systemic inflammatory conditions. Recombinant human antithrombin Ills (ATllls) developed in our lab inhibit thrombin and fXa with comparable efficiency to endogenous ATIII, but bind heparin with 50x greater affinity and are 10x more resistant to neutrophil elastase cleavage. These "super-beta-ATIN" molecules also target heparin-coated surfaces of an in vitro flow model at least 9x more efficiently than does plasma ATIII. Therefore, early intervention via super-beta-ATIII loading of thrombin/fXa inhibitory activity onto native vascular and heparin-coated biomaterial surfaces may prove generally useful for blocking surface-catalyzed thrombotic, occlusion and restenosis events. As these pathologies develop under widely varying rheological conditions, flow-related factors modulating ATIII surface loading in different vessels and implant applications must be elucidated. Goals of this R21 are to obtain rheological and dosing data to support clinical translation efforts. Aim 1 will investigate mass transport effects on super-beta-ATIII surface delivery under low-vs-high and steady-vs-pulsatile flow, using simulations, an in vitro flow model, and lumenal surface analysis of rabbit blood vessels. Aim 2 will evaluate the ability of super-beta-ATIII to prevent thrombosis in rabbit carotid shunt and stenosis models.